If you're asking about how computers represent negative numbers in binary, then it's, as @MaryMarvel stated, by using the two's complement notation.
Two's Complement Notation is used to represent signed binary numbers. The Two's Complement of a number is all its bits reversed, plus one. (i.e: The Two's Complement of A is NOT A + 1)
So -50 for example is represented as follows: 11001110
The Left-most bit is the sign bit, when it is 1 the number is negative.
So how to convert 11001110 to its decimal form?!
It's done as follows: 1st) Get the two's complement of it, that is:
00110001 + 1 = 00110010
2nd) if you convert 00110010 to its decimal form you'll get 50 and since the number is negative then the answer is -50
hope you got it

You should be told if you're dealing with an unsigned or signed number. If signed then it will be half of an unsigned. I.E., a byte is 256 bits, so a signed byte would be -128 to 127. An unsigned would be 0-255.